1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to systems and methods for pumping solid-state slab lasers.
2. Description of the Related Art
Doped-insulator slab lasers are solid state lasers that are used in a variety of applications requiring moderate to high optical output power. The slab lasing material is typically comprised of a host crystal doped with an ion, such as, for example, ytterbium doped yttrium aluminum garnet (Yb:YAG). High power slab lasers employing lasing media with high aspect ratio slab configurations have traditionally been optically pumped through the broad slab faces with one or more linear flashlamps and have been cooled either by forced convection or conduction through the same faces. (Pumping is the process by which an active (lasing) medium is excited to achieve a population inversion. The population inversion is a condition by which energy is stored in the medium with sufficient gain to cause the medium to lase. See Solid-State Laser Engineering, Second Edition by Walter Kocchner, published 1988 by Springer-Verlag, Berlin, Germany.)
Face pumping has been necessary due to the low brightness of the flashlamp pumping sources, which have precluded pumping through the smaller area ends and edges of the slab. Face cooling is advantageous in high aspect slab lasers to minimize the conduction path through the lasing medium for thermal energy produced by intrinsic and extrinsic nonradiative processes within the medium (quantum defect, quenching, excited state absorption and/or up conversion). Minimizing the thermal conduction path is important to minimize the average temperature and temperature gradient within the lasing medium, as is discussed later. Because they require optical pumping and cooling through the same slab faces, the traditional flashlamp-pumped slab lasers are necessarily complicated in their design, requiring optically transparent cooling means.
More modern slab lasers are optically pumped by narrow band, high brightness laser diode arrays. The higher brightness of these laser diode pump sources relative to flashlamps allows a high aspect ratio slab to be pumped either through the narrow edges of the slab in directions generally transverse to the laser beam or through the narrow ends of the slab in directions generally collinear with the laser beam. Edge and end pumping of the slab allows the faces to be cooled without constraining the cooling system to also transmit the pump beam into the slab, thereby simplifying the design. The pumping configuration that results in the optimum absorption and distribution of pump energy in the lasing medium is preferred.
A configuration capable of achieving both high absorption and uniform distribution of pump energy in an edge-pumped geometry is described in commonly assigned patents entitled Laser Pump Cavity Apparatus with Integral Connector and Method, issued Apr. 25, 2000 to R. W. Byren et al., U.S. Pat. No. 6,055,260 (Attorney Reference No. PD 970064 and referred to hereinafter as the ""064 application) and Laser Pump Cavity Apparatus with Improved Thermal Lensing Control, Cooling, and Fracture Strength and Method, issued Oct. 26, 1999 to R. W. Byren et al., U.S. Pat. No. 5.974,061 (Attorney Reference No. PD 970226 and referred to hereinafter as the ""226 application), the teachings of both of which are incorporated herein by reference.
The approach described in the ""064 application requires a cladding layer formed in a hyperbolic cylindrical shape that is thicker at the edge of the slab than in the center to obtain the proper optical concentrator performance. If the outer surface of the cladding layer is cooled to a constant heat sink temperature, the difference in thermal conductance across the width of the slab due to the change in the cladding thickness produces a nonuniform temperature gradient within the slab. This, in turn, introduces nonuniform thermal lensing and stress birefringence, which are difficult to compensate.
In addition to improving pump efficiency and uniformity, it is essential to efficiently remove the large amount of heat that is generated within the lasing medium.
An increase in the operating temperature within the lasing medium reduces the population inversion that can be achieved for a given level of pumping, thereby reducing efficiency. Reducing the operating temperature of the laser increases the gain and extraction efficiency. More specifically, reducing the operating temperature increases the stimulated emission cross-section of the active lasing medium. This lowers the saturation fluence of the active lasing region, which makes it easier to extract the stored energy for gain-switched and Q-switched systems, without damaging the optical coatings at the exit surfaces of the bulk lasing material. Similarly, reducing the temperature also lowers the saturation intensity, which makes it easier to extract power for continuous and high pulse rate systems without optical damage.
Temperature gradients cause mechanical stress within the lasing medium. When the medium is stressed, the crystal becomes birefringent, and energy in the laser beam if polarized in a direction that is neither along nor orthogonal to the stress gradient will be converted from the desired polarization to an undesired polarization as the beam propagates along the beam axis through the crystal. This induced birefringence is undesirable for many applications. For example, when the crystal faces are cut at the Brewster angle to extract energy of a desired polarization, energy converted to an orthogonal polarization will be internally reflected, resulting in a loss of output efficiency.
As another example, in a typical multi-pass master oscillator power amplifier laser system that uses a straightforward polarizer and 90xc2x0 polarization rotation means to separate the master oscillator input beam from the amplified output beam, depolarization of the beam due to thermal stress induced birefringence in the amplifier will cause a portion of the output beam to feed back into the master oscillator, potentially damaging the oscillator components, reducing the output power, and imprinting on the output beam a nonuniform intensity profile which adversely affects beam quality. It is therefore desirable to maintain a one-dimensional temperature gradient within the slab and orient the polarization of the beam to be collinear with or orthogonal to this gradient in order to avoid depolarization due to thermal stress birefringence. Temperature gradients also cause refraction or bending of the laser beam as it enters, propagates through, and exits the lasing medium. Physical distortion of the lasing medium due to nonuniform thermal expansion produces a lensing effect at the entrance and exit surfaces of the lasing medium. The index of refraction of the medium, which is a function of both the temperature and stress within the medium, varies across the beam producing graded-index lensing within the medium. If the temperature gradient is one dimensional within the slab, i.e. isotherms are parallel to slab faces, the thermal lensing effects can be compensated by means available in the present art. For example, conventional cylindrical lenses can be used to provide a first order correction. The beam can also be guided by total internal reflection at the faces, as described in the above-mentioned co-pending applications, minimizing the beam spreading within the slab. It is, therefore, desirable to maintain a one-dimensional temperature gradient within the slab in order to permit thermal lensing compensation by available methods.
In side-pumped laser cavity configurations, heat is removed from the lasing medium by cooling mechanisms applied to the broad faces of the slab. Prior art methods for cooling the broad slab faces include air cooling, liquid cooling systems (forced convection and impingement) and conductive cooling through metal heat sinks. Air cooling is limited to lower power lasers due to relatively poor thermal transfer. Liquid cooling requires careful sealing arrangements to prevent leakage that would contaminate the diode pump arrays and associated relay optics and cause optical damage to surfaces exposed to the laser beam.
Similarly, the performance of prior art methods that utilize direct contact of metal heat sinks to the solid state pump cavity medium has been less than desirable. Differences between the thermal conductivity and thermal expansion coefficients of the metal and solid-state pump cavity medium result in inadequate thermal transfer rates and significant mechanical stress. The above-mentioned co-pending applications describe a composite slab structure with top and bottom cladding layers that are diffusion bonded to the slab-shaped active lasing region. These cladding layers are shaped having outer cylindrical optical focusing surfaces which concentrate the pumplight entering from the edge of the composite slab, thereby providing efficient and uniform pumping across the slab.
Because the thickness of the cladding layers varies across the slab, direct cooling of the slab through the outer cylindrical focusing surfaces, either by liquid or solid conductive means, wherein the cooling surface is maintained at a constant temperature, produces a non-uniform temperature gradient across the active lasing region resulting in a non-uniform thermal lensing and birefringence condition which is difficult to correct externally.
Edge cladding regions may be used to improve the optical performance of the concentrator adding to the efficiency and uniformity of pumping. These edge cladding regions, however, provide a thermal conduction path through the edges of the slab which exacerbates the non-uniform temperature gradient near the ends of the slab active lasing region.
Thus, there was a need for improved methods and apparatus for cooling a slab laser and controlling the direction of heat flow within the lasing medium to increase operating efficiency and minimize thermally-induced birefringence and lensing. This need was addressed by U.S. Pat. No. 6,014,391 issued Jan. 11, 2000 to R. W. Byren and entitled Thermally Improved Slab Laser Pump Cavity Apparatus. with Integral Concentrator and Method of Making Same (Attorney Reference Number PD 970508 and referred to hereinafter as the ""508 application), the teachings of which are incorporated herein by reference. This application provides a solution to the temperature nonuniformity problem by adding thermal resistance between the slab and the heat sink through a variable thickness compliant thermal interface layer between the cladding surface and the cold plate heat sink.
A problem arises with this approach at high pumping levels in that the total temperature drop across the cladding and thermal interface layers can be quite large, resulting in high temperatures within the active lasing region of the slab. For quasi-four level laser media such as Yb:YAG, the gain (stimulated emission cross-section) of the medium decreases rapidly with temperature due to thermal population of the lower lasing level, degrading the performance of the laser. Also, thermal conductivity of materials such as YAG decreases with temperature, exacerbating the temperature rise problem.
Hence, a need remains in the art for a system or method for improving the performance of pumping arrangements for slab lasers. More specifically, a need remains in the art for a system or method for reducing the temperature drop across the cladding layers of side and end pumped high energy slab lasers to improve the thermal conductivity between the lasing medium and the heat sink thereof.
The need in the art is addressed by the concentrator and method of the present invention. In a most general implementation, the inventive concentrator includes a volume of at least partially transmissive material and a plurality of facets disposed at at least one surface of the material. Each of the facets is disposed at a position-dependent angle relative to the surface effective to cause an internal reflection of energy applied to the layer whereby the density of the applied energy varies as a function of position.
In the illustrative implementation, the volume is an active medium, i.e., a slab. The slab has substantially parallel, planar upper and lower surfaces and first and second edges therebetween. A plurality of cladding layers are disposed on the upper and lower surfaces of the slab. The facets are provided in the cladding layers on the upper and lower surfaces of the slab and angled as a function of distance relative to the first or the second edge. The facets provide a Fresnel reflecting surface or a binary optic surface and the facet angles as a function of distance relative to the slab edge are approximately:
xcfx86(x)=0.5xcex1m2t xmax(1xe2x88x92x/xmax)/[1xe2x88x92xcex1m2xmax2(1xe2x88x92x/xmax)2]
where:
xcex1m=bulk absorption coefficient (cmxe2x88x921)=xcex1m=cos xcex80/xmax;
xcex80=internal injection angle (radians);
xcfx86(x)=facet angle as a function of distance from slab edge (radians);
t=slab thickness (cm); and
xmax=point at which 100% of pumplight is absorbed, i.e., center of slab (cm).